classes: Combine rootfs_<pkgtype> and populate_sdk_<pkgtype>

classes: Combine rootfs_<pkgtype> and populate_sdk_<pkgtype>

There is no real reason to have these separate files any longer. It does
mean in meta-toolchain type recipes some extra variables are defined but
it also means the common code can be grouped and maintained together
which I believe is more beneficial. We therefore merge the classes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>